Clinical and morphologic findings are described in a 22-year-old man with prolonged thrombocytosis and coronary and splenic arterial thrombi causing myocardial and splenic infarcts. The absence of preexistent extensive coronary atherosclerosis, the presence of thrombus in more than one epicardial artery and in multiple intramural coronary arteries, the presence of arterial thrombosis in a non-coronary artery (splenic), and the absence of another apparent cause of the arterial thromboses are evidences that the intraarterial clotting in our patient was related to the severe thrombocytosis.